Gas detection at low temperature and low power consumption is a major concern for the field of chemical sensors primarily used in the context of wireless detection applications. In many gas sensing industrial applications, for example, metal oxide based chemo-resistors such as those based on SnO2 can be used for the detection of reducing gases (e.g., H2, CO, CH4) and oxidizing gases (e.g., NOx). The electrical resistance of SnO2 gas sensors, for example, can increase in the presence of an oxidant gas, due to a charge transfer reaction between the NOx gas and metal oxide, which includes the removal of electrons from the metal oxide. On the other hand, in presence of a reducing gas, the electrical resistance of the same SnO2 based sensor decreases due to a charge transfer reaction that supplies the chemo-resistor with electrons.
Depending on the type of gas, such sensors typically require high operating temperatures up to 450° C., which can cause the sensor to become a high-consumer of electric power typically in a consumption range of 30-200 mW. In MEMS (Micro-electro-mechanical System) gas sensor applications, the thermal isolation of a suspended membrane supporting a heated sensing layer is very high, but the power consumption can hardly be limited below 20 mW, for example, for producing a temperature of 400° C. Such levels of power consumption are considered high for some applications, even very high for wireless gas sensing applications.
Another important gas chemical sensor is a “pellistor”, where a gas is detected due to an exothermic catalytic reaction with a heated surface, which further increases the temperature of the catalytic surface. This temperature increase further increases the resistance of a metallic resistor used for heating the surface. For example, a simple pellistor can be prepared from Al2O3 containing Pd catalysts, which covers the Pt resistor used for heating the catalyst material. Due to their intrinsic principle, the pellistors again consume an increased amount of electrical power, which may not be accepted in many future applications, including the wireless gas sensors.
In one prior art, exemplary polymeric film materials used on a SAW/BAW chemical sensor include, but are not limited to, polyisobutylene, polyphenylenesulphone, polyacrilic acid, polystyrene, polystyrene sulfonated, ethyl cellulose, polyethyleneimine, polyanilines, polyvinylpyrollidone, Teflon, Mylar, Kaladex, polyethylene adipate, polyethylenemaleate, polycaprolactone, polyethyleneglicols, polyepichlorohydrine, phenyl-methyl polysiloxanes, perfluoro-2,2 dimethyl 1,3 dioxole (PDD), polypyrrole, etc. The interactions between sensitive polymeric, film and target molecules (gas molecules) include: π-π stacking, electrostatic, hydrogen bonding, size/shape recognition, van der Waals, acid-base.
In the prior art it has been demonstrated that analytes can be detected with a SAW/BAW chemical sensor. Such analytes can include, for example, but are not limited to non-polar vapours (hexane, toluene, octane), polar vapours (acetone, methanol), chlorinated hydrocarbons such as tetrachloroethylene (PCE), trichloroethylene (TCE), vinyl chloride (VC), carbon dioxide, carbon monoxide, ozone, nitric oxide, hydrofluoric acid, hydrogen sulphide, sulphur dioxide-, and so forth.
The organic sensing films utilized in a gas sensing application must be chemically and mechanically stable and can be applied onto the respective device surfaces by methods compatible to industrial standard coating procedures. The organic sensing films are typically deposited by spin-coating (in the case of polymer materials), or by evaporation (e.g., organic vapor phase deposition or in the case of small organic molecules). Similarly, the organic film is deposited on the entire surface and subsequently removed from the region where it is not necessary.
Based on the foregoing it is believed that a need exists for the realization of the chemical sensors operating at low temperatures, or even at room temperature. Additionally, a need exists for the synthesis and deposition of organic thin films-based gas chemical sensors operating at room temperature based on supra-molecular chemistry. Finally, a need exists for new and low-cost techniques for SAW chemical sensor fabrication, without the need for lithographic processes, which are expensive and can waste a great amount of material, which should be removed by etching from the areas where it is not required. It is believed that these and other problems can be solved by the solutions discussed herein, which generally relates to a SAW chemical sensor, wherein all the layers (e.g., metallic layer, dielectric layer, and/or functionalized sensing layer) for the fabrication of the sensors are accomplished by an additive processing technique, such as direct printing.